


a thousand miles down to the seabed

by avosettas



Category: Undertale (Video Game)
Genre: Bad Sanses Poly/Dream, Crosstale Sans (Undertale), Dissociation, Dreamtale Nightmare Sans (Undertale), Dreamtale Sans | Dream (Undertale), Dusttale Sans (Undertale), Egg Laying, Horrortale Sans (Undertale), Hurt/Comfort, Killer Sans (Undertale) - Freeform, Leviathan AU, M/M, Mating Cycles/In Heat, Multi, Nesting, Panic Attacks, bad sanses poly - Freeform, past captivity
Language: English
Status: Completed
Published: 2021-02-26
Updated: 2021-02-26
Packaged: 2021-03-17 00:40:10
Rating: Mature
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,765
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29708991
Author URL: https://archiveofourown.org/users/avosettas/pseuds/avosettas
Summary: The first thought that surfaces in Cross's mind upon waking isHide, hide,hide.It's absurd, of course, and he knows it. He's perfectly safe in the grotto, curled a short distance from where the rest of his shiver sleeps, cushioned in Nightmare's tentacles. Even Nightmare is asleep, upper body slumped on the edge of the deep trench to rest beside the pile the others have made.
Relationships: Sans/Sans (Undertale)
Comments: 11
Kudos: 219





	a thousand miles down to the seabed

**Author's Note:**

> based on [this thread](https://twitter.com/Askellie_ut/status/1364773170234888192)
> 
> the leviathan au is by [sku!](https://twitter.com/skumhuu)

The first thought that surfaces in Cross's mind upon waking is _Hide, hide, **hide**_. 

It's absurd, of course, and he knows it. He's perfectly safe in the grotto, curled a short distance from where the rest of his shiver sleeps, cushioned in Nightmare's tentacles. Even Nightmare is asleep, upper body slumped on the edge of the deep trench to rest beside the pile the others have made. 

Ignoring the fact that Cross hadn't intended to fall asleep, everything is perfectly fine. But even when he blinks a few times, swishes his tail to rid himself of the remnants of grogginess, his mind still feels foggy. 

_Hide, hide, **hide**_ -

He's already gone when his brain catches up to his instincts, following the usual path he and Dream use from the grotto to the shallower sea floor. He can't imagine that none of the others noticed - every swish of his tail sends a hail of bubbles flying behind him, and it takes conscious effort to force it to slow to something less conspicuous. 

The rocky floor of the deep transitions to soft sand, and part of Cross just wants to curl up on it. He didn't notice before, so focused on following his instincts, but everything feels _off_ and his belly hurts. 

He tries to ignore it, but he knows what it is. Every movement reminds him of his shakiness, and even in the cool waters of the seabed his bones are burning. 

It's when he gathers the first set of plastic rings that he can't deny it any longer. He forgets what it'd been called, but it was miserable. Too shaky and feverish to move (nevermind that the first time, he'd been left in a shallow tank, restrained and _on his back_ anyway) and in so much pain. 

Gaster had never done anything after running tests and restraining him the first time, never told him anything. All he'd done the second time, and the third time, and each and every time after that was cluck his tongue and tell _Project X_ how disappointed he was that his research would be delayed once more. 

(Cross hadn't been Cross, not back then. Just Project X.) 

As he gathers another piece of human garbage into his arms, Cross supposes that dealing with whatever this is away from Gaster is the most he can hope for. It's fine, he tells himself, wedging a broken, dull-edged bottle into the mess he's carrying. It's fine, because he won't be near Gaster, and he can get this stupid thing over with, whatever it is. 

His arms are too full to carry anything else, now. It's disgusting. 

He swims towards the outcrop that divides the slow, slow incline of the sandy floor. The seaweed at the edges is itchy against his feverish scales, even worse as he darts between the crevices, but it's still softer than the garbage in his hands. 

Finally, one crevice opens to a cave, shallower than he'd like. But it will do, or so Cross tells himself, dumping the garbage in his hands into the very depths of it, mentally marking off its location before turning and swimming out once more. 

For more garbage. 

It makes his stomach roil to even think about - or maybe it's just the situation, he isn't sure. Eventually, he has a veritable pile of plastic and dull glass and even some driftwood in the back of the cave. 

He doesn't want to do this. But he forces his shaking hands to shove the garbage together as his instincts force him to, reasoning that the sooner he gets this over with, the sooner he can return to Dream and the rest of his shivermates. 

There isn't even any seaweed on the floor of this cave, and as Cross settles himself into the pile of garbage, it makes him want to cry. His instincts are screaming, but he can't figure out what it is he needs. He supposes Gaster took all his chances of finding out away from him, keeping him so isolated behind a glass wall, rather than in the ocean with other mers _like he should have been_. 

His belly cramps, and Cross hisses at the pain. The first time this had happened - no, the second time, but the first time he'd been left alone for it - it had taken a week before he'd laid something, too soft and broken and _wrong_ to be eggs. It had been awful. 

(The worst part might have been how Gaster had gotten rid of them right in front of him, though. Like they were _garbage_ , and he was just an object.) 

Cross resolves not to wait for it to happen, this time. With a hiss, he forces his hemipenes out, and though it's satisfying when he brushes his cloaca with the movement, something in his instincts scream _wrong, wrong, wrong!_

His instincts can have the victory, though it's not like he was planning on touching himself anyway. The… eggs couldn't be laid if the opening of his cloaca was blocked off. 

He's not really aware of himself, for a while - only the pain and the the scent of his own blood, from where his claws dig into his scales. Dimly, he registers the lessening cramps, and the fact that he's probably stinking up the cave - possibly the whole outcrop - with pheromones, but he can't bring himself to care. 

When the pressure in his belly is finally gone, he takes a moment to breathe. The garbage beneath him digs into his back unpleasantly, and his gills brush up against bits of plastic and glass when they flare. And his slit burns, which feels just as bad - maybe worse - than the cramps had been. 

He wants to sleep. Wants to curl up with his shivermates in Nightmare's tentacles, and sleep for days, unheeding of the sun, because the only light that ever reached the grotto was that which was brought by Nightmare and Dream. He thinks he could sleep for days, and he knows that they would let him. 

But he has to do this now, before he goes back. 

His mind screams at him as he forces his claws into the soft, broken shells of the eggs. They're so _wrong_ \- he can't place why, he _knows_ they are, but he doesn't know why. He doesn't know what he's done wrong, only that it's over. 

It's over, and as soon as he finishes this, he can go _home_. 

(He doesn't know when he began to see the grotto as home, just like he doesn't know when his shiver became _his shiver_.) 

Every motion of his hands makes him want to sob, though, and even as he tries to be _done_ with it all, his vision blurs. His claws are covered in bits of shell and pieces of plastic that he's accidentally torn up, and all he can do is wonder what is _wrong_ with him. 

He sits in the pile of garbage, staring at the remnants of it all longer than he can say. His hands hover uselessly over the clump of crushed up eggs, twitching with an unconscious urge that he has no name for. 

Until the water around him stirs. 

A group of pilot fish, small and white and practically glowing, swim about his humeri in tight circles, nearly hitting the bones. It take Cross's slow mind too long to put it together, and by then, Dust has already entered the cave. 

He stops, eyes flickering downwards at Cross's _mess_ , and for a split second disappointment is visible on his face. But then it drops into its usual apathy once more, and he swims forward even as Cross presses himself to the cave wall. 

"Why'd you do that?" he asks in his raspy voice, and Cross can hear how carefully he's trying to keep any inflection from the question. 

Cross can only shake his head, flinching when the other pulls him close and towards the mouth of the cave. Dust is smaller than he is, but even so he pulls Cross gently through the water, holding him tightly against his ribcage. Cross is all but useless, clutching at him like a lifeline, tail dragging in the sand. 

Dust murmurs something, but he doesn't quite hear it. Then, it doesn't matter; they're out of the cave, in the open waters above the outcrop. Horror is waiting, and a little ways away, Killer is swimming frantic circles, though he immediately stops when he sees Cross. 

Someone else speaks, and Cross is passed to one of the others. He's too exhausted to tell who it is. 

He lets himself fall asleep, safe in his shivermate's arms. 

~

"...broken? Why would he break his own eggs?" 

Cross wakes to Dream's voice, soft and worried. He opens his eyes, but everything is bright for a moment, and he closes them once more. He can't tell where he is. 

Something shifts - beneath him? Around him? The voices are quieter, now, though when he tries to open his eyes, it's no darker. It takes him a moment to realize that what he's lying on is a cushion of Nightmare's tentacles, and the bright light is from his eye. 

"Did you sleep well?" he asks, drowning out what little Cross can still hear of Dream's conversation with the others. A lone tentacle comes up to him, stroking along his skull gently. 

Nightmare narrows his eye when Cross doesn't speak, instead averting his eyes. Everything still hurts, and he's barely even processed Nightmare's question, but at least the heavy feeling in his stomach is gone. 

"Cross," Nightmare says, soft and dangerous. "Tell me what happened." 

The tentacle that had been hovering near Cross's skull skitters down his neck, and it makes him jolt. 

(Like he's been shocked.)

"Is this… a scar?" Nightmare sounds so angry, and that's _bad_. Cross has known that he is a useless failure; he's known the rest of them would realize it eventually. 

He didn't realize how upset he'd be on the day that finally happened. 

Though the first tentacle still hovers near his neck, a second comes up, and strokes near his gills, as if to remind him that they're there, before settling around the end of his tail, like a lover's embrace. "Breathe," Nightmare encourages him. 

His gills flare and everything hurts. Why does everything hurt when he'd gone through all that to make it _stop_?

"Breathe," Nightmare repeats. 

He can't wrap his head around how thankful he is that Nightmare doesn't have him in his hand. Isn't flipping him onto his back, like he does with the others. 

(It's just to groom them. Cross knows this. But the single time Nightmare had tried to take care of him, well… Cross doesn't really remember what exactly had happened afterwards.) 

"I -" Cross begins, but the words are choked. He doesn't want to think about what happened to him to get here, no matter how much he prefers being in Nightmare's care to Gaster's. 

Someone settles beside him, the pressure of their body against his own warm and welcome. "Don't push him," Dream chides his brother as he snuggles up closer to Cross. 

Nightmare only snorts at him, tentacles curling idly around him as they had Cross. "Yes, brother." 

Dream sighs, then turns to face Cross. His hands are warm on Cross's face as they cup his jaw, and his sockets narrow as he stares searchingly. "Are you okay?" 

"I - Yes. No." Cross looks away, but Nightmare is on one side, and on the other side, the rest of the shiver has drifted closer, hovering near the outskirts of the coil of tentacles. He looks down instead, where his tail is curled up, but it's almost worse; despite having forced himself to be done as quickly as possible, his slit is very clearly an angry purple, the edges agitated and injured. "I don't know," he finally decides, feeling far away from himself. 

The water shifts again and Nightmare mutters something in an irritated tone as the others make themselves at home, as close to Cross as they can. He doesn't begrudge them the opportunity to comfort their shivermate, though, instead curling his tentacles tighter around them. 

"Gave us a scare," Killer says, and though he laughs a bit, there's no humor in it. "...Y'know, if you didn't wanna spend your heat with any of us, all y'had to do was say." 

Cross looks up and stares at him, too dumbfounded to continue feeling sorry for himself. "My… what?" he asks, only realizing he sounds like an idiot when the others stare. 

"...what'd you… think was happening?" Horror asks after a moment, and it's easier to respond to him than anyone else, simply because Cross can't see his face. 

"I don't know," he replies miserably, staring at a bright scale on Dream's hip. It's a pretty shade of pink; he hadn't noticed, because at first glance all he'd ever noticed was yellow-orange. "...I just - I thought - I had to make something. To - to lay them in." 

Nightmare finally speaks. "Cross. Did you…" He sounds uncertain. Nightmare _never_ sounds uncertain, and that's all Cross can focus on. Until he speaks again: "Did you force yourself to lay them so early?" 

Cross blinks. Early? 

(They had always come out like that, soft and broken and _wrong_ , no matter if he waited or if he forced them.) 

"Did you force yourself to lay them?" Nightmare amends. The uncertainty is gone from his tone; all that's left is patience. So Cross nods. 

"Why the hell would you do that?!" Killer interrupts.

"They - that's," Cross stammers. Killer's empty sockets burn with rage, and Cross can't tell if it's directed at him. "He always - they were always destroyed anyway. And they - they were wrong no matter if I waited." 

"Who destroyed them?" 

"That's enough, Killer." Nightmare shoos Killer away with his hand, provoking a pitiful chirp. But Cross answers him anyway, though it takes him a few moments. 

"The - Gaster. The doctor," he mumbles, looking down, focusing on the feeling of Dream's hands, still on his face. "It - he left me alone. When that happened. And it - I hated being left alone more than I hated - I hated the tests so much. But at - I didn't like when he left me alone." It's a pitiful, stuttered explanation. Full of half-sentences and unfinished thoughts. 

"You were in captivity," Dream concludes softly, frowning sadly when Cross nods. "Oh, Cross." 

"I - it wasn't -" 

"Don't say it wasn't that bad," Dust grunts. The ends of his scarf trail against Cross's ribs softly. "'Cause obviously it was. Boss can't even groom you without sendin' you into a panic." His skull clinks gently against Cross's shoulder as his hands shift in his pockets - likely clenching and unclenching his claws in anger. 

"I'm sorry," Cross says helplessly. 

"Cross," Nightmare says, his voice an authoritative rumble. "None of that is your fault." 

"I'm sorry," he repeats. 

"'S okay," Killer slides up against him once more, voice placating. "Just means _we_ get to teach you about everything. Like heats. And hunts." His grin is wide. "I, for one, am looking forward to helping you through your next heat, if you get my drift." 

"Killer," Nightmare warns, but Cross just relaxes in his embrace. 

"Sounds nice," he says quietly, safe in the knowledge that they won't abandon him. Dream's hands slip from his face as he leans closer, purring as he snuggles into Cross's chest.

"...should rest again," Horror tells him, resting his jaw on top of Cross's skull. "Y'look… exhausted." 

But there's a niggling fear in his mind still. "What - what if I can't learn?" he blurts before he can stop himself, exhaustion and heat ruining his mental filter. 

Killer and Dust both laugh, though it sounds as tired as Cross feels. It's Nightmare who responds, though, fondness oozing through his commanding tone: "You're mine now, Cross, and I take care of what is mine."

"Believe me," Dream says sleepily, barely raising his head from Cross's chest. "I'm not sure they'd let you leave if you tried." 

"If he _wanted_ to, we would," Dust argues with a roll of his eyelights. "But he doesn't _want_ to, he thinks we're gonna make him leave. Which we're not, obviously." 

The verbal confirmation is all he needs to relax once more into the impromptu cuddle pile, but Dream's purr on his chest, and Killer and Dust's tails against his own, and Horror's bones beside his leave him downright serene. 

He's practically asleep already by the time Nightmare folds a couple of tentacles over them, a warm blanket against the cold of the deep.

**Author's Note:**

> twitter @avosettas


End file.
